1. Field of the Invention
The present invention is in the field of vibration attenuation and isolation, and in particular, relates to a passive skyhook-type isolation system.
2. Description of the Prior Art
Isolating a structure or system from an unwanted source of vibration is a classic problem in mechanical engineering. There are many machines that benefit from vibration isolation. Common examples include automobiles, trains, heavy machinery, and most recently, spacecraft launch vehicles. One objective of vibration isolation is to reduce the vibration transmission from an external disturbance to a sensitive system. This is typically accomplished by the addition of a suspension, including spring and damper elements, which reduces the sensitive system's response to external disturbances. Isolation systems are usually designed to attenuate either shock or persistent harmonic excitations.
Design and implementation of passive isolation systems has been studied for many years. There are inherent limitations on the amount of isolation that can be provided using passive elements. Design of passive isolation systems involves a trade-off of resonant response and high-frequency attenuation. A solution referred to as a skyhook damper provides damping at resonance without increasing transmissibility at high frequencies (Karnopp, D., Crosby, M. J., Harwood, R. A., "Vibration Control Using Semi-Active Force Generators, J of Engineering for Industry, 96, pp. 619-626, 1974). However, skyhook damping requires that a viscous damper be connected to an inertial reference frame, which is not practical in most situations. There has been work to realize a skyhook damper actively using sensors, actuators and control electronics (Fuller, C. R., Elliott, S. J., and Nelson, P. A., Active Control of Vibration, Academic Press, New York, 1996). Other realizations of skyhook damping devices use semi-active approaches (Rakheja, S., "Vibration and Shock Isolation Performance of a Semi-Active `On-Off` Damper," J of Vibration, Acoustics, Stress, and Reliability in Design, 107, pp. 398-403, 1985). While these studies have yielded promising results, they often introduce the need for a power source and necessarily add complexity to the isolation system.
U.S. Pat. No. 5,558,191 is a passive tuned mass damper using a secondary mass connected to the primary structure by a spring and a viscoelastic element suspension system. The suspension system is tuned to the resonant frequency of the primary structure, damping the primary structure motion by the movement of the secondary mass. The primary mass is not isolated by a spring/damping system, as in the present invention, but is rigidly connected to something (the earth) that transmits vibrations. It is designed to minimize movement at resonance, not to yield low transmissibility at frequencies above resonance.
It is an object of the present invention to present a virtual skyhook isolation system that is able to demonstrate the desired transmissibility characteristics using an entirely passive approach. Like the skyhook damper, it has the benefit of limiting vibration amplitude at resonance without increasing transmissibility at higher frequencies. This is accomplished without the need for an inertial reference frame by using a secondary tuned spring-mass-damper system. Unlike other tuned mass-damper implementations that generally provide vibration attenuation at structural resonance, the concept presented herein focuses on improved high frequency isolation characteristics along with increased damping at the primary mass resonant frequency.